X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-08-05
2001-07-31
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000
Reexamination Certificate
active
06269141
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a computer tomography apparatus which includes a scanning unit which includes a radiation source and a detector unit which is connected thereto in order to detect a conical radiation beam, emitted by the radiation source, after its passage through an examination zone or an object present therein, a drive device for producing a relative motion in the form of a helix, consisting of a rotation about an axis of rotation and an advance in the direction parallel to the axis of rotation, between the scanning unit and the examination zone or the object, and a reconstruction unit for reconstructing the spatial distribution of the absorption within the examination zone from the measuring data acquired by the detector unit within a detector window defined by the helix.
A computer tomography apparatus of this kind (also referred to as CT apparatus hereinafter) is known from the PCT application SE 98/00029, dated Jan. 14, 1998. For the reconstruction of the absorption distribution the known computer tomography apparatus takes into account only the measuring data which is present within a detector window which is defined in the direction of the z axis by the projection of two successive turns of the helix (in this context and hereinafter a detector window is to be understood to mean the part of the measuring surface of the detector unit which acquires exclusively the data required for the reconstruction). It can be demonstrated that when the detector window is configured in this manner, the radiation source projects each voxel in the examination zone, upon its entry into and its departure from the radiation beam, onto the detector window from exactly 180° offset positions (relative to the respective voxel). The measuring data thus acquired enables exact reconstruction of the absorption distribution in the examination zone, even when the object present therein is longer than the part of the examination zone which is covered by the conical radiation beam.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to improve a computer tomography of the kind set forth even further. This object is achieved according to the invention in that the connecting lines from the radiation source to the two edges of the detector window, being mutually offset in the direction of the axis of rotation, intersect two segments of the helix which are offset by the distance (2n+1)p in the direction of the axis of rotation, where n is an integer number≧1 and p corresponds to the axial offset between two neighboring turns of the helix.
Whereas in the known computer tomography apparatus the two edges of the detector window are situated at a distance from one another which corresponds to the projection of two neighboring detector turns on the detector unit, according to the invention this distance amounts to an odd multiple thereof. As a result, when the projection of a point in the examination zone has moved from one edge of the window to the other edge, the radiation source will have rotated exactly through an angle (2n+1)&pgr; about this point. In this case no redundant measuring data is obtained either. All measuring data acquired within the detector window can be used for the reconstruction.
Because of the fact that the radiation source rotates through an odd multiple about the individual points of the examination zone, the sensitivity of the method to data inconsistency (resulting from motion of the object to be examined during the scanning) is less than in the known method. This is so even though the signal-to-noise ratio, averaged across the entire examination zone, in principle is not worse than during the acquisition of the measuring data by means of the known computer tomography apparatus (provided that the speed of advancement and the intensity of the radiation source are the same in both cases).
However, in comparison with the known computer tomography apparatus the signal-to-noise ratio at individual points is more uniformly distributed across the examination zone, because the location-dependent fluctuation of the periods of time during which a point is projected onto the detector window is less severe. In the known computer tomography apparatus a variation of the projection duration of 2:1 occurs when the radius of the object cylinder, within which an object can be completely covered by the conical radiation beam, amounts to half the radius of the trajectory of the radiation source about the axis of rotation. For the same geometry in an apparatus according to the invention this variation amounts to only 1.25:1 (for n=1) or 1.14:1 (for n=2). The artefacts accompanying this variation are thus substantially reduced by the invention.
The detector window according to the invention can be realized by shaping the detector unit and/or the conical radiation beam formed by a collimator accordingly. When the detector unit describes an arc of circle around the axis of rotation, the development of the detector window should have the shape of a parallelogram; a distorted parallelogram would be required if the detector unit were to define (in a plane perpendicular to the axis of rotation) an arc of circle about the radiation source. The realization of detector units with such shapes of the development is intricate.
However, use can also be made of a detector unit whose development is shaped as a rectangle, provided that this rectangle is configured to be so large in the direction of the axis of rotation that it encloses the development of the detector window. Such a rectangular detector, therefore, must be larger than the desired detector window. In accordance with the invention the ratio of the required detector surface to the actual detector surface, however, is more favorable than for the detector window of the known CT apparatus.
A preferred embodiment is disclosed in claim
2
. In that case the distance between the edges of the detector window thus amounts to three times the distance between two turns of the helix.
Claim
3
describes a preferred embodiment. The advantages thus achieved are analogous to those which occur in conventional computer tomography apparatus in which only a single detector row is included and the center of the detector elements and the puncture point of a line intersecting the axis of rotation are mutually offset by one quarter of the detector width; after half a rotation of the scanning unit, the detector unit has been displaced by one half detector width in the line direction. Therefore, the same data is not measured twice but in intermediate positions; the measuring data is thus more attractively distributed and the image quality is enhanced. Such an offset is not possible in the known computer tomography apparatus.
Similar effects are also obtained in the direction of the axis of rotation in the embodiment described in claim
4
.
The embodiment of the invention as defined in claim
5
enables a choice between a (first) mode of operation, in which the radiation source rotates through the angle &pgr; about each examination point during its passage through the radiation cone, and a (second) mode of operation in which this rotation amounts to (2n+1)&pgr;. When the same rotary speed is used for both modes of operation, in the case of the same detector dimensions the scanning speed in the first mode of operation is a factor 2n+1 higher than in the second mode of operation in which, however, the signal-to-noise ratio or the image quality is better. The user can thus select the mode of operation which is most attractive for the relevant examination.
Claim
6
discloses a preferred embodiment for the reconstruction of the absorption distribution in the examination zone from the measuring data acquired within the detector window. The absorption distribution can also be reconstructed from the measuring data in a different manner, but the described means enable particularly simple reconstruction steps and a particularly high quality of the reconstruction or a particularly high image qu
Proksa Roland
Timmer Jan
Bruce David V.
Hobden Pamela R.
U.S. Philips Corporation
Vodopia John F.
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